1. Field of the Invention
The present invention relates to a viewfinder optical system of simple form suited to be used in cameras or the like.
2. Description of Related Art
In the viewfinder optical system, an image of an object to be photographed is formed on the focusing screen, which is observed by looking through the eyepiece lens. For the eyepiece lens, until now, many simple configurations have been proposed.
(i) What has been known as the most common configuration ever is the combination of a positive lens of relatively low dispersion and a negative lens of relatively high dispersion cemented together to form a cemented lens of positive refractive power.
(ii) Recently, to reduce the production cost, the eyepiece lens simplified to only one plastic lens has come to find its use in many viewfinders.
(iii) Further, to reduce the weight of the camera and achieve a much more desired reduction of the production cost, the optical system for forming an erecting image is taken into consideration, and an assembly of reflecting mirrors is employed instead of the heavy prism of glass material.
Such a mirror type optical system for forming an erecting image takes a longer optical path with a decrease of the finder magnification. To compensate for this, the eyepiece lens has to be constructed with two lenses of positive and negative refractive powers. This type of viewfinder has been proposed in, for example, Japanese Laid-Open Patent Applications No. Hei 2-181713 and No. Hei 6-235870.
(iv) Meanwhile, as the optical system using a diffractive optical surface, mention may be made of, for example, Japanese Laid-Open Patent Application No. Hei 6-324262, in which the diffractive optical surface is used in a part of a photographic optical system for the purpose of further improving the correction of aberrations.
As another example of application of the diffractive optical surface to an observation optical apparatus, mention may be made of, for example, U.S. Pat. No. 5,446,588, in which the eyepiece comprises three lenses. At least one of their lens surfaces is made to be a diffractive optical surface. With this diffractive optical surface, various aberrations including spherical aberration are further improved to insure that the eye relief becomes longer and, at the same time, the apparent field of view becomes wider. The eyepiece lens disclosed here is of high performance.
(v) Yet another recent trend in the field of art of lens shutter cameras is to improve the compact form of the objective lens system. To cope with this trend, there is a growing demand that the external viewfinder to be incorporated in the camera, too, be of compact size. To achieve realization of a compact viewfinder, not only the total number of constituent lenses must be reduced, but also the refractive power of each of these lenses must be increased. As a result, it becomes difficult to obtain a high optical performance. To remedy this, the choice of glass materials in making up the lens elements may be considered, so that the ranges of refractive indices and dispersions are widened.
However, as disclosed in Japanese Laid-Open Patent Application No. Hei 9-105863, for the optical parts of the external viewfinder such as that in the lens shutter camera, on consideration of the reduction of the cost and the productivity, plastic materials are often used. Hence, it becomes of great importance to achieve improvements of the performance and compact form of the optical system without sacrificing the cost and productivity.
The various conventional examples (i) to (v) described above have their respective individual problems as follows.
(a) As, in the prior known technique (i), the eyepiece lens is constructed with the positive lens of relatively low dispersion and the negative lens of relatively high dispersion which are cemented together to form a cemented lens of positive refractive power, the difference in dispersion between these lenses is utilized to correct mainly lateral chromatic aberration. With this, diffusion of colors at points of high contrast in the image of the object to be photographed, or at the field-of-view frame, is reduced, thus realizing a viewfinder which is comfortable to view. However, for such a construction, it is substantially impossible to largely reduce the cost in producing the eyepiece lens.
(b) Then, as in the prior known technique (ii), the viewfinder is recently constructed with one plastic lens. Since this is amenable with the injection molding manufacture technique, the production cost is remarkably reduced. From the demand for reduction of the production cost, the opportunity of using this technique is increasing. However, as a matter of course, with such a construction, as far as the ordinary or refractive optical system is concerned, chromatic aberrations become substantially impossible to correct, thus lowering the comfortability of looking through the viewfinder.
(c) The prior known technique (iii) is to construct the optical members for forming an erecting image with reflecting mirrors and to construct the eyepiece lens with two lenses. In such a type of viewfinder optical system, if the 2-component eyepiece is constructed in the combination of a positive lens of relatively low dispersion and a negative lens of relatively high dispersion, correction of chromatic aberrations becomes possible. However, if the two constituent lenses of the eyepiece are made up from equivalent plastic materials in order to reduce the production cost of the camera, correction of chromatic aberrations becomes difficult.
For example, acrylic resin is available as the plastic material of relatively low dispersion and polycarbonate resin as the plastic material of relatively high dispersion. So, if these materials are used in the positive and negative lenses, respectively, it becomes possible to correct the chromatic aberrations of the viewfinder optical system. However, polycarbonate resin is slightly weak in mechanical strength. If a counter-measure to such a problem, the advantage in cost is diminished. So, it is not suited to be used.
(d) The prior known technique (iv) using the diffractive optical surface is adapted to the optical system having a large number of lens members and has an aim to further improve the correction of all aberrations. That is, when correcting the aberrations of the light rays, it has been the common practice to use the diffractive optical surface as one degree of freedom. Although the diffractive optical surface can be used for a similar purpose of reducing the number of lens members to that in using the aspheric surface which is well known in the art, it may otherwise be considered, as a method of more effectively adapting the application of the characteristics of the diffractive optical surface, that the use of the diffractive optical surface is limited to the correction of chromatic aberrations alone.
(e) In the prior known technique (v), the eyepiece lens of the viewfinder optical system is not required in general to have a particularly large apparent field of view and a particularly long eye relief. Even with the eyepiece lens in the form of a single lens, all aberrations except chromatic aberrations leave little residual, being corrected so much as to be negligible in many cases.
As the criterion for determining whether or not the viewfinder is comfortable to look through, the magnitude of the residual chromatic aberrations is adopted. If this quantity is large, color diffusion appears at the contour of an object to be observed, or flare components are produced, lowering the quality of the viewfinder image.
In general, the ordinary or spherical lenses or like refractive optical elements constitute a real image viewfinder and its optical performance is improved by arranging a pair or pairs of lens units of positive and negative refractive powers. To correct the various aberrations to a minimum and in good balance, the number of lens members is caused to increase. Introduction of an aspheric surface is advantageous at reducing the number of lens members in such a manner as to keep the optical performance. As far as the chromatic aberrations are concerned, however, for the correcting purposes, determination must be made of which two of the lens members of positive and negative refractive powers are selected to be paired with their materials in desired dispersion difference. Even by introducing an aspheric surface, no effect of correcting the chromatic aberrations can be expected.